System and method for measuring power of optical signals carried over a fiber optic link

ABSTRACT

A pilot tone generator receives optical energy from an optical communication medium carrying a plurality of optical signals. Each optical signal carries data modulated at a unique wavelength and further modulated with a unique identification signal. The identification signal has an amplitude corresponding to an optical power of the associated optical signal. The pilot tone receiver detects each identification signal from the optical energy received and determines its corresponding amplitude. The pilot tone receiver calculates the optical power of each optical signal in the optical energy in response to the amplitude of the associated identification signal.

BACKGROUND OF THE INVENTION

[0001] In a wavelength division multiplexing (WDM) optical system, it isdesirable to measure optical powers of individual optical signalstransported along a fiber optic link. Conventional methods of performingsuch power measurements require expensive components to separate theoptical signals transported in the fiber prior to power measurement sothat each signal may be measured individually. Not only are theyexpensive, these optical components tend to be physically bulky and addto the considerations during management of the fiber optic link.

SUMMARY OF THE INVENTION

[0002] From the foregoing, it may be appreciated by those skilled in theart that a need has arisen for a technique to measure power of opticalsignals transferred over a fiber optic link. In accordance with thepresent invention, a system and method for measuring power of opticalsignals carried over a fiber optic link are provided that substantiallyeliminate or greatly reduce disadvantages and problems associated withconventional optical power measurement techniques.

[0003] According to an embodiment of the present invention, there isprovided a method for measuring power of optical signals carried over afiber optic link that includes receiving a plurality of optical signalsfrom the fiber optic link with each optical signal including anidentification signal modulated therewith. An amplitude of eachidentification signal received is determined and an optical power ofeach optical signal is determined in response to the amplitude of eachidentification signal received.

[0004] The present invention provides various technical advantages overconventional optical power measurement techniques. For example, onetechnical advantage is in the simultaneous measurement of optical powerof a plurality of optical signals without separating the optical signalsfor individual measurement. Another technical advantage is in the use ofless costly and reduced number of optical components since only themodulations of many optical signals are detected and analyzed. Yetanother technical advantage is to adjust a detection bandwidth accordingto receiver position in the network, signal to noise ratio of receivedoptical signals, and/or desired accuracy. Other technical advantages maybe readily ascertainable by those skilled in the art from the followingfigures, description, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0005] For a more complete understanding of the present invention andthe advantages thereof, reference is now made to the followingdescription taken in conjunction with the accompanying drawings, whereinlike reference numerals represent like parts, in which:

[0006]FIG. 1 illustrates a simplified block diagram of an opticalnetwork;

[0007]FIG. 2 illustrates a simplified block diagram of a pilot tonereceiver in the optical network;

[0008]FIG. 3 illustrates a process flow diagram for measuring opticalpower of optical signals received at the pilot tone receiver andtransported in the optical network.

DETAILED DESCRIPTION OF THE INVENTION

[0009]FIG. 1 is a simplified block diagram of an optical network 10.Optical network 10 includes a transmitter 12 that receives data inputsI₁-I_(n). Transmitter 12 includes a plurality of laser and pilot tonemodulator units 12 a-12 n. Laser and pilot tone modulator unit 12 agenerates an optical signal 14 a at a unique predetermined wavelength λ₁to transport information received at data input I₁. Laser and pilot tonemodulator unit 12 a also modulates a unique identification signal, orpilot tone, ID₁ onto its generated optical signal 14 a. Similarly, otherlaser and pilot tone modulator units, such as 12 n, generate opticalsignals 14 n where the associated data input I_(n) is modulated at aunique predetermined wavelength λ_(n) and is modulated with a uniqueidentification signal ID_(n).

[0010] Optical network 10 includes a combiner 15 operable to receive aplurality of optical signals 14 a-14 n and to combine those signals intoa multiple wavelength signal 16. As one particular example, combiner 15may be a wavelength division multiplexer (WDM). Optical network 10communicates multiple wavelength signal 16 over an optical communicationmedium 20. Optical communication medium 20 may have a plurality of spans20 a-20 n of fiber, each with an optical amplifier 22 or other types ofoptical elements including an optical add/drop multiplexer, an opticalcross connect unit, signal conditioning devices, and/or lossy elements.One type of optical element used in optical network 10 may be an opticaltap 24. Optical tap 24 allows for a distant location to capture aportion of the transmission carried by optical communication medium 20.

[0011] Optical network 10 also includes a separator 26 operable toseparate individual optical signal 14 a-14 n from multiple wavelengthsignal 16. Separator 26 can communicate individual signal wavelengths orranges of wavelengths to a bank of receivers 28 a-28 n and/or otheroptical communication paths. Separator 26 may be, for example, awavelength division demultiplexer (WDM). Receivers 28 a-28 n receiverespective optical signals 14 a-14 n for decoding in order to recoverthe original signal as a respective data output O₁-O_(n).

[0012] In order to manage optical network 10, it is desirable to measurethe optical power of each optical signal 14 a-14 n carried over opticalcommunication medium 20. The present invention contemplates the use ofidentification signals ID₁-ID_(n) to measure the optical power of theircorresponding optical signal 14 a-14 n.

[0013]FIG. 2 shows a simplified block diagram of a pilot tone receiver30 used to measure optical power of optical signals 14 a-14 n. Thefunctions performed by pilot tone receiver 30 may be performed inhardware, software, or a combination of both. Pilot tone receiver 30receives a portion of the optical transmission from opticalcommunication medium 20 through optical tap 24. Optical tap diverts aportion of the optical energy from optical communication medium 20 topilot tone receiver 30. In the example shown, optical tap 24 extracts 5%of the optical energy from optical communication medium 20 though otherpercentages of extraction may be incorporated as desired.

[0014] Pilot tone receiver 30 receives the extracted optical energy ofoptical signals 14 a-14 n from optical tap 24 at an optical/electricalconverter 32. Optical/electrical converter 22 converts the opticalenergy into electrical signals. The electrical signals are fed to ananti-alias filter 34 for removal of the high frequency component of theelectrical signals. The anti-aliased electrical signals are thencombined at a combiner 36 with random noise from a dither source 38 toimprove the signal to noise quality of the original signals. Theimproved electrical signals are then converted into digital form by ananalog to digital converter 40.

[0015] A filtering and down sampling unit 42 performs several functionson the digital signals received from analog to digital converter 40.Band pass filters may be used in order to isolate the frequencies ofinterest in the digital signals. The data rate of the digital signalsmay also be down sampled to minimize processing, allow long timestorage, and allow narrow detection bandwidths. Filtering and downsampling unit 42 then detects for each identification signal ID₁-ID_(n),either sequentially or in any desired order through changing ofidentification signal detection coefficients, and measures its amplitudefor processing by an optical power processor 44. Optical power processor44 stores information from the detected identification signal in aworking storage 46 to perform the appropriate processing and coordinateswith information about pilot tone receiver 30 determined at manufactureand stored in a calibration storage 48. Optical power processor 42determines an optical power of an associated optical signal from theamplitude of its identification signal. The measured optical power maythen be used to adjust any amplifier gains within optical network 20 asdesired.

[0016] Identification signals ID₁-ID_(n) may be of a variety of typesprovided that, when detected, their amplitude is proportional to theoptical power of the associated optical signals 14 a-14 n. Amplitudemodulation is one technique for providing the appropriateproportionality. It is also preferable for identification signalsID₁-ID_(n) to not interfere with one another during transport anddetection. With no interference, the optical power of many opticalsignals may be measured simultaneously without individually separatingout the optical signals. This requirement can be accomplished throughsine wave amplitude modulation with different frequencies for each ofidentification signals ID₁-ID_(n). Since only a small amount of theoptical energy is extracted by optical tap 24 from optical communicationmedium 20, only small amplitudes of modulation are used foridentification signals ID₁-ID_(n). As an example, a 4% amplitudemodulation may be performed for identification signals ID₁-ID_(n). Byusing small amplitudes of modulation, identification signals ID₁-ID_(n)do not interfere with the data traffic carried by optical signals 14a-14 n. Through sine wave amplitude modulation detection, the opticalpower for a given optical signal 14 is determined as follows:

P=R/(M*G),

[0017] where P is the optical power to be measured,

[0018] R is the amplitude of the received identification signal,

[0019] M is the index of modulation used to modulate the optical sourcewith the identification signal, and

[0020] G is the gain of the modulation receiver.

[0021] The accuracy of the measurement of P depends on the accuracy ofeach of R, M, and G. The accuracy of R, the amplitude of theidentification signal, relates to the signal to noise environment at thedetection point. At a location with many optical signals 14 a-14 npresent, a noise density level N per Hertz is controlled by theaggregate of these optical signals 14 a-14 n. The noise in the detectionbandwidth B then becomes N*B. The ratio of signal S to noise in thedetection bandwidth is thus S/(N*B). If an arbitrarily small opticalsignal 14 is present at the location, it may have an arbitrarily lowsignal to noise density ratio S/N. To achieve a specified accuracy forR, it will be required to achieve a minimum signal to noise ratio in thedetection bandwidth. Thus, for small signals S, bandwidth B will beminimized to achieve a specified accuracy for R.

[0022] The accuracy of M, the modulation index, is dependent upon howprecisely the source modulation is known and how the modulation indexchanges during optical signal 14 propagation. A technique for preciselycontrolling M at the optical source can be found in copending U.S.patent application Ser. No. 09/567,576 filed May 10, 2000 and entitled“Method and Apparatus for Maintaining a Pre-determined Ratio of a PilotTone Power and a Mean Optical Output Power of an Optical Signal” whichis hereby incorporated herein by reference. The variation of themodulation index during propagation is mostly dependent on an amount ofamplified spontaneous emission noise included in the optical channelmeasurement. The amount of amplified spontaneous emission noise isrelated to optical bandwidth. However, with good optical carrier tonoise ratios where bit error rates are less than 10⁻¹², the variation ofthe modulation index with propagation is generally negligible.

[0023] The accuracy of G, the modulation receiver gain, depends upon howwell this parameter is known. Pilot tone receiver 30 provides aquantifiable output R sensitive to a particular identification signalusing various optical and electronic components. The major uncertaintyof G is in the variability of the operating characteristics of eachoptical and electrical component from one unit to the next, especiallythe unit to unit variability of optical taps 24. Being unit to unitrelated, this variability can be measured in conjunction with allcomponents of pilot tone receiver 30 at time of manufacture and includedas calibration data stored in calibration storage 48. The accuracy of Gthen becomes dependent upon how well it is measured at the time ofmanufacture and if it drifts with time and environment during use.

[0024] As seen in pilot tone receiver 30, detection of R occurs in thedigital domain and in the program domain. This allows for an ability tovary the detection bandwidth. By being able to vary the detectionbandwidth, pilot tone receiver 30 may be optimized for the signal tonoise ratio present at a particular detection point within opticalnetwork 10. As optical signals propagate through optical network 10, thesignal to noise ratio seen by pilot tone receiver 30 changes as opticalchannels are added or dropped from any given optical span 20 a-20 n. Fora given accuracy of R at different detection points within opticalnetwork 10, different detection bandwidths may be implemented. Also, ifat a given detection point a different accuracy of R is desired, thedetection bandwidth may be varied to accommodate the new accuracyrequirement. Variation of the detection bandwidth may be performed on anoptical signal by optical signal basis.

[0025] With sine wave amplitude modulation used for identificationsignals ID₁-ID_(n), an example limit on the narrowness of the detectionbandwidth may be the sum of the phase noise of the modulationtransmitter 12 and the phase noise at the frequency reference of pilottone receiver 30. The phase noise of the modulation transmitter 12controls how wide the frequency is for the identification signalID₁-ID_(n) modulation. The phase noise at the frequency reference ofpilot tone receiver 30 controls a minimum detection bandwidth. Anadditional limitation on the narrowness of the detection bandwidth isthe amount of space allocated in working storage 46 to perform filteringat the detection bandwidth. Signal to noise ratio improves as moreinformation is accumulated and stored for processing. As bandwidthbecomes small, the sample time desired increases. Storage requirementsfor longer sample times is larger than for shorter sample times. Highsample rates may require a relatively large amount of storage space.Down sampling performed by filtering and down sampling unit 42 slows therate that information leaves analog to digital converter 40 so that lessstorage space is needed for processing. This limitation may beinsignificant if sufficient storage space can be provided in pilot tonereceiver 30.

[0026] One of the requirements for components within optical network 10relates to reliability. Certain components that carry large numbers ofwavelengths and thus large amounts of data traffic should be designedwith high reliability characteristics. Reliability in electronic systemscan be maximized in several ways. One way is to minimize hardware andthe other way is to minimize software. Lots of hardware or lots ofsoftware are well known to lead to reliability problems. By selectingthe component interface for traffic critical components at the junctionbetween filtering and down sampling unit 42 and optical power processor44, reliability is maximized without any compromise to accuracy. Anycomponent interface selected prior to analog to digital converter 40 canlead to degraded accuracy due to the added complexity of conveying ananalog value across the boundary. A component interface between analogto digital converter 40 and filtering and down sampling unit 42 leads toless reliability due to the relatively high data rate for data acrossthis boundary. Selecting the component interface after optical powerprocessor 44 has less reliability due to the inclusion of software andthe electronics associated with the processing of the power measurement.

[0027]FIG. 3 shows an example process flow diagram for measuring powerof an optical signal. The process begins at block 50 where each opticalsignal 14 a-14 n is modulated with a unique identification signal. Eachidentification signal ID₁-ID_(n) is modulated with an amplitudeproportional to an optical power of its associated optical signal 14a-14 n. The optical signals 14 a-14 n are multiplexed for transmissionacross optical communication medium 20 at block 52. A portion of theoptical energy transmitted across optical communication medium 20 isextracted at block 54 by optical tap 24. At block 56, the optical energyis converted to electrical signals. At block 58, the high frequencycomponents within the electrical signals are removed. At block 60, thesignal to noise ratio of the electrical signals is improved. A digitalrepresentation of the electrical signals is generated at block 62. Atblock 64 the detection bandwidth is determined. At block 66, filteringis performed according to the detection bandwidth. The data rate of thedigital representation is down sampled to a lower rate at block 68. Anidentification signal is detected at block 70 and its amplitude isdetermined at block 72. An optical power of the associated opticalsignal is calculated at block 74 in response to the amplitude of theidentification signal. The process is repeated for each optical signaland identification signal pair carried by optical communication medium20. In this manner, optical power of an optical signal is determinedwithout having to process any of the data carried by the optical signal.

[0028] Thus, it is apparent that there has been provided, in accordancewith the present invention, a system and method for measuring power ofoptical signals carried over a fiber optic link that satisfies theadvantages set forth above. Although the present invention has beendescribed in detail, it should be understood that various changes,substitutions, and alterations may be made herein. For example, pilottone receiver 30 may include other or fewer functions than those shownand described and still measure the optical power of an optical signalusing a detected amplitude of its identification signal. Other examplesmay be readily ascertainable by those skilled in the art and made hereinwithout departing from the spirit and scope of the present invention asdefined by the following claims. Moreover, the present invention is notintended to be limited in any way by any statements or any example madeabove that is not otherwise reflected in the appended claims.

What is claimed is:
 1. A method for measuring power of optical signalscarried over a fiber optic link, comprising: receiving a plurality ofoptical signals from the fiber optic link, each optical signal includingan identification signal modulated therewith; determining an amplitudeof each identification signal received; determining an optical power ofeach optical signal in response to the amplitude of each identificationsignal received.
 2. The method of claim 1, wherein the identificationsignals do not interfere with data carried by the optical signals. 3.The method of claim 1, wherein each identification signal does notinterfere with any other identification signal.
 4. The method of claim3, wherein each identification signal is sine wave amplitude modulatedto its respective optical signal.
 5. The method of claim 1, wherein onlya portion of the total energy of the plurality of optical signals isreceived.
 6. The method of claim 1, wherein the optical power of eachoptical signal is proportional to the amplitude of its respectiveidentification signal.
 7. The method of claim 1, further comprising:down sampling a data rate of the optical signals to provide narrowdetection bandwidths.
 8. The method of claim 1, further comprising:adjusting a detection bandwidth at a detection point of the opticalsignals.
 9. The method of claim 8, wherein the detection bandwidth isdetermined in response to a phase noise of a transmitter used inmodulating the identification signals onto the optical signals and aphase noise of a frequency reference used in receiving the opticalsignals.
 10. The method of claim 1, further comprising: determining anamount of accuracy required on the amplitude of each modulation signal.11. A system for measuring power of optical signals carried over a fiberoptic link, comprising: a filtering and down sampling unit operable toreceive a digital representation of optical signals carried by anoptical transmission medium, the filtering and down sampling unitoperable to identify an identification signal associated with aparticular one of the optical signals, the filtering and down samplingunit operable to determine an amplitude of the identification signal; anoptical power processor operable to receive the amplitude of theidentification signal from the filtering and down sampling unit, theoptical power processor operable to determine an optical power of theparticular optical signal in response to the amplitude of theidentification signal.
 12. The system of claim 11, wherein the filteringand down sampling unit is operable to adjust a detection bandwidth usedin identifying the identification signal.
 13. The system of claim 12,wherein the filtering and down sampling unit is operable to decrease adata rate of the digital representation to narrow the detectionbandwidth.
 14. The system of claim 11, wherein the identificationsignals for the optical signals do not interfere with each other toallow the optical power of the optical signals to be measuredsimultaneously.
 15. The system of claim 11, further comprising: acalibration storage having calibration data used by the optical powerprocessor in determining the optical power of the optical signals.
 16. Acomputer readable medium having code for measuring power of opticalsignals carried over a fiber optic link, the code operable to: receive aplurality of optical signals, each of the plurality of optical signalshaving a unique identification signal modulated therewith, each uniqueidentification signal having an amplitude corresponding to an opticalpower of its associated optical signal; adjust a detection bandwidth foreach unique identification signal according to a desired powermeasurement accuracy; identify each unique identification signal withinits associated detection bandwidth; determine the amplitude for eachunique identification signal; calculate an optical power of each opticalsignal from the determined amplitudes.
 17. The computer readable mediumof claim 16, wherein the code is further operable to: down sample a datarate associated with the plurality of optical signals in order to narrowthe detection bandwidth.
 18. The computer readable medium of claim 16,wherein the code is further operable to: limit a width of the detectionbandwidth in response to a phase noise associated with transmission ofthe plurality of optical signals and a phase noise associated withreceipt of the plurality of optical signals.
 19. The computer readablemedium of claim 16, wherein the code is further operable to: vary thedetection bandwidth in response to a signal to noise ratio at a point ofreceipt of the plurality of optical signals.
 20. The computer readablemedium of claim 16, wherein the code is further operable to: calculatethe optical power of the optical signals in response to a desiredaccuracy of the identification signal amplitudes, a desired accuracy ofthe modulation index associated with the identification signals, and adesired accuracy of the gain at a detection point of the identificationsignals.